Heh, at this point, my guppies have bred with endlers livebearers, and have turned into real mutts. I've got a stable breeding population in my 30 gal, and a larger population in my 125. The 125 has an arrow, which makes them feeder guppies, but I've noticed the arrow allways catches the males with the big tail displays first. That bright flag of color that slows down their swimming makes them a tasty target.
The exceptional mutts that look really striking, I try to pull from the main tank and put in the 30 gal. The dull boring ones that breed in the 30 gal go back into the main tank to get eaten. I've been doing this process for a year, and I still don't have a 30 gallon full of showcase guppies. I think they just mongrel too widely. But it keeps me busy...